A novel flow battery—A lead acid battery based on an electrolyte with soluble lead pptx

The influence of additives Ahmed Hazza, Derek Pletcher∗, Richard Wills The School of Chemistry, The University, Southampton SO171BJ, UK Received 16 November 2004; received in revised for

A novel flow battery—A lead acid battery based

on an electrolyte with soluble lead(II)

IV The influence of additives Ahmed Hazza, Derek Pletcher∗, Richard Wills

The School of Chemistry, The University, Southampton SO171BJ, UK

Received 16 November 2004; received in revised form 24 January 2005; accepted 31 January 2005

Available online 22 March 2005

Abstract

During development of an undivided flow battery based on the Pb(II)/Pb and PbO2/Pb(II) couples in aqueous methanesulfonic acid, it was noted that battery performance might be improved by additives that (i) decrease the roughness of the lead deposit at the negative electrode and (ii) enhance the kinetics of the Pb(II)/PbO2couple at the positive electrode This paper reports the study of sodium ligninsulfonate and polyethylene glycol as potential levelling agents for lead and of three inorganic ions as possible catalysts for the Pb(II)/PbO2 couple The addition of 1 g dm−3ligninsulfonate leads to uniform deposits without the tendency to form dendrites but leads to a slight decrease in both charge and energy efficiency for the battery Only nickel(II) reduced the overpotential for PbO2deposition but again it has an adverse influence

on the energy efficiency

© 2005 Elsevier B.V All rights reserved

Keywords: Flow batteries; Lead acid; Methanesulfonic acid

1 Introduction

In recent papers[1–3], a novel flow battery has been

re-ported The battery is based on the electrode reactions of

lead(II) in methanesulfonic acid (see Eqs (1)–(3) in the

pre-vious paper[3]) The reactions differ from those in the

tradi-tional lead acid battery[4,5]because lead(II) is highly soluble

in methanesulfonic acid[6]

During these earlier studies, two potential roles for

ad-ditives were identified Firstly, it was found that although

deposits of PbO2at the positive electrode always appeared

to have the essential uniformity, under some conditions the

lead deposit at the negative electrode became very uneven

and there was the possibility of the metal growing across the

interelectrode gap, hence shorting of the battery Secondly,

it was noted that the poor kinetics of the PbO2/Pb2+couple

DOI of original article:10.1016/j.jpowsour.2005.01.048.

∗Corresponding author Tel.: +44 2380 593519; fax: +44 2380 593781.

E-mail address: dp1@soton.ac.uk (D Pletcher).

led to overpotentials during both charge and discharge that were large compared to those at the negative electrode and the IR drop through the electrolyte The overpotentials at the positive electrode were therefore the major cause of loss in energy efficiency during battery cycling Hence, both level-ling agents for lead at the negative electrode and catalysts for the positive electrode reaction were of interest Clearly, any additive must be soluble and stable in the very acidic electrolyte Moreover, in addition to achieving their objec-tive, successful additives must also not influence adversely the behaviour of the other electrode since this battery is in-tended to operate without a separator For example, additives designed to smooth the lead deposit must also be stable to oxidation at a PbO2anode

A number of additives have been reported for levelling

of lead electroplates from acid electrolytes [7–9], although these have been used only for the deposition of relatively thin electroplates in quite different conditions from those in a flow battery Because of their solubility and stability in acidic media, sodium ligninsulfonate and polyethylene glycol were 0378-7753/$ – see front matter © 2005 Elsevier B.V All rights reserved.

doi:10.1016/j.jpowsour.2005.01.049

selected for study A number of inorganic ions (e.g Sb(V),

Bi(III), Fe(III), Ni(II) and Ag(I)) are known to influence

the conductivity and electrocatalytic properties of PbO2

([10–12]and references therein[13,14]) Moreover, Cu(II) is

a common additive in recipes for the electroplating of PbO2

layers [15–17], although its role is not clear and alloying

elements in lead positive electrode current collectors in lead

acid batteries are known to improve battery performance

(Ni(II), Fe(III) and Bi(III)) in the electrolyte on the battery

performance was also investigated

2 Experimental

All solutions were prepared with water from a Whatman

Analyst Purifier, methanesulfonic acid (Aldrich) and lead

carbonate (BDH) and where appropriate sodium

ligninsul-fonate (Aldrich), polyethylene glycol, molecular weight 200

(Lancaster Synthesis), nickel carbonate (BDH), ferric nitrate

(Hogg Laboratory Supplies) and bismuth nitrate (BDH) All

solutions were thoroughly deoxygenated with a rapid stream

of nitrogen bubbles prior to experiments

Voltammetry was carried out in a two compartment, glass

cell with a volume of∼20 cm3 and it was immersed in a

Camlab W14 water thermostat at a temperature of 298 K

The vitreous carbon rotating disc electrode (area 0.08 cm2) or

rotating nickel disc (0.32 cm2) working electrodes and the Pt

wire counter electrode were in the same compartment but the

saturated calomel reference electrode (SCE) was separated

from the working electrode by a Luggin capillary The cell

was designed with a fine glass frit to allow efficient entry of

gases to the solutions Before each experiment, the rotating

disc electrodes were polished with alumina powder (1␮m,

then 0.3 and 0.05␮m) on a moist polishing cloth (Beuhler),

wiped on a clean piece of polishing cloth and rinsed well with

deionized water after each polish

The charge/discharge experiments were carried out in a

small undivided flow cell with two electrodes, geometric

areas 2 cm2 and interelectrode gap either 4 or 16 mm [2]

The electrodes consisted of a carbon powder/high density

polyethylene composite back plate (core), thickness 3.2 mm,

with an active layer (tile) on the surface produced by heat

bonding under pressure Three types of electrodes with

dif-ferent active layers were used:

(i) Type 1 electrodes were fabricated by pressing a piece

of 40 ppi nickel foam, initial thickness 1.8 mm into the

plate with a pressure of 6 kg cm−2at 433 K.

(ii) Type 2 electrodes were fabricated by pressing a piece

of 70 ppi reticulated vitreous carbon, initial thickness

1.5 mm into the plate in the same conditions

(iii) Type 3 electrodes were prepared from type 2 by

scrap-ing away the reticulated carbon layer with a knife This

leaves a rough surface with many vitreous carbon

parti-cles

Scanning electron microscope (SEM) photographs of these structures have previously been reported[2] The electrolyte temperature was controlled at 298 K

Electrochemical experiments were carried out using one

of three sets of equipment: (i) a laboratory constructed po-tentiostat controlled by a PC with a National Instruments LabVIEWTM 5.1 interface card and in-house written soft-ware, (ii) a laboratory constructed galvanostat controlled by the PC with a Measurement Computing CIO-DAS-08/JR 12-bit interface card, (iii) a model 263A EG & G potentio-stat/galvanostat coupled to a PC via a National Instruments MC-GPIB interface card; the system was controlled and data recorded using the EG & G M270 software package The ro-tation rate of the disc electrodes (RDEs) was controlled with

an EG & G Model 616 RDE unit Deposits on the electrodes were examined employing a Phillips ESEM environmental scanning electron microscope including elemental analysis

by EDAX Deposit thicknesses were estimated using the fa-cility to tilt the sample within the ESEM chamber in order to examine the edge They were also estimated using Faraday’s law and assuming 100% current efficiency for the deposition

3 Results and discussion

3.1 Additives for lead electrodeposition

vitre-ous carbon) negative electrode after extensive charging in the flow cell with an electrolyte initially containing 1.5 M Pb(CH3SO3)2+ 0.9 M CH3SO3H In fact, the cell with an in-terelectrode gap of 4 mm had shorted after 3 h when a charge

of 216 C cm−2 had passed and it had been dismantled for

examination Some deposition of lead had occurred through-out the carbon foam but there was an accumulation at the

Fig 1 Scanning electron micrograph of a type 2 (reticulated vitreous car-bon) negative electrode after charging at 20 mA cm −2for 3 h Initial

elec-trolyte: 1.5 M Pb(CH 3 SO 3 ) 2 + 0.9 M CH 3 SO 3 H without any additive Elec-trolyte flow rate: 10 cm s −1.

Fig 2 Cyclic voltammogram at a Ni RDE (1600 rpm) in a solution of 4 mM

Pb(CH 3 SO 3 ) 2 + 2 M CH 3 SO 3 H + 1 g dm −3sodium ligninsulfonate

Poten-tial scan rate: 50 mV s −1 The inset shows a comparison on a more sensitive

current scale of the forward scans for electrolytes with and without

lignin-sulfonate.

surface closest to the counter electrode Moreover, an

ex-tensive network of dendrites can be seen growing from the

front face of the reticulated vitreous carbon Clearly, a

lev-elling agent would be advantageous The quality of the lead

deposit depends on a number of experimental parameter,

es-pecially the deposition charge, current density, the form of

the electrode, and whether the electrode has previously been

charge/discharge cycled For example, the deposits reported

as Figs 4 and 5 in[2]are much superior The sample shown

asFig 1was deliberately selected to illustrate the possible

need for an additive

3.1.1 Sodium ligninsulfonate

A series of cyclic voltammograms were recorded at a Ni

RDE (1600 rpm) in a solution of 4 mM Pb(CH3SO3)2+ 2 M

CH3SO3H + 1 g dm−3 sodium ligninsulfonate.Fig 2

illus-trates the typical response for a voltammogram run over the

potential range where lead deposition and re-dissolution is to

be expected The inset compares the forward scans for

solu-tions with and without additive on a more sensitive current

scale While both solutions show waves for the reduction of

Pb(II) to Pb on the scan towards more negative potentials, the

presence of the ligninsulfonate clearly causes some increase

in the nucleation overpotential for the deposition of lead (in

fact from∼35 to ∼60 mV) and the reduction wave to

be-come significantly more drawn out along the potential axis

The half wave potential is shifted negative by the addition

of ligninsulfonate but at sufficiently negative potential, the

limiting currents are the same and proportional to the square

root of the rotation rate of the disc, confirming mass

trans-port control In both solutions, a ‘nucleation loop’ is clearly

seen and the potential where the current is zero on the back

scan (a reasonable estimate of the formal potential for the

Pb2+/Pb couple in the two solutions) is close to−470 mV,

both with and without additive On the other hand, the slope

of the voltammogram through the formal potential is much

lower with ligninsulfonate present reflecting the slower

kinet-ics of electron transfer when the additive is in the solution Both solutions give a sharp lead dissolution peak positive to the formal potential and the ratio of charges associated with the dissolution and deposition of lead are very close to one Pulsed current experiments confirmed that the overpotentials for the deposition and dissolution of lead are increased by the presence of ligninsulfonate but the magnitudes of the overpotentials remain very small compared with the other voltage losses in the battery We would also note the ratio of ligninsulfonate to Pb(II) is much higher in the voltammetric experiments than in the battery electrolyte

Using solutions containing 0.3 M Pb(CH3SO3)2+ 2.0 M

CH3SO3H with and without 1 g dm−3 sodium

ligninsul-fonate, lead layers were plated onto the Ni RDE (900 rpm) at

a series of current densities and the deposits were examined

by electron microscopy At low current densities, the deposits were relatively smooth even without additive At high current densities, the levelling action of the ligninsulfonate becomes apparent.Fig 3compares the deposits at two current densi-ties; these pictures are all taken with the sample at an angle

so that the thickness of the deposit and the quality of the edge can be assessed With a current density of 50 mA cm−2,

the ligninsulfonate changes the deposit from one made up of large boulders of lead to a relatively smooth deposit Even

at 375 mA cm−2, the influence of ligninsulfonate remains

strong and the deposit changes from an uneven, angular and dendritic structure to a uniform ‘cauliflower’ form likely to

be quite acceptable in battery operation

Therefore, it can be concluded that ligninsulfonate is a levelling agent for lead in the battery electrolyte The next stage was to examine whether it affects the positive elec-trode chemistry Voltammograms were recorded with a vit-reous carbon disc electrode over the range 0 to +1900 mV with a scan rate of 50 mV using solutions containing 1.5 M Pb(CH3SO3)2+ 0.9 M CH3SO3H and with various additions

of sodium ligninsulfonate The cyclic voltammograms all had the general features for the deposition and dissolution

of PbO2 as reported previously[1] On the other hand, ad-ditions of ligninsulfonate led to a progressive increase in the overpotentials associated with both nucleation and deposi-tion of lead dioxide as well as a slight decrease in charge balance in the deposition and reduction cycle For example,

in experiments containing 1 g dm−3ligninsulfonate the

over-potential associated with PbO2 deposition was 157 mV at

5 mA cm−2(cf 120 mV in the absence of ligninsulfonate)

and the charge efficiency was 78% (cf 88%) At least in part, the decrease in charge efficiency may be due to the oxy-gen evolution current during PbO2deposition becoming rel-atively more important as the PbO2deposition current den-sity decreases Scanning electron micrographs showed that the form and quality of PbO2layers on vitreous carbon was unaffected by the presence of sodium ligninsulfonate and de-posits were almost structureless when viewed on a 10␮m

scale

The influence of sodium ligninsulfonate on the structure

of the lead and lead dioxide deposits was also studied in a

Fig 3 SEM pictures of the edges (tilted at 70 ◦) of Pb deposits onto a Ni RDE (900 rpm) from baths containing 0.3 M Pb(CH

3 SO 3 ) 2 + 2 M CH 3 SO 3 H with (b and d) and without (a and c) 1 g dm −3sodium ligninsulfonate Current densities: 50 mA cm−2 (a and b) and 375 mA cm−2 (c and d) Deposition time:

600 s.

small flow cell[2], in these experiments fitted with two

car-bon powder/high density polyethylene composite plate

elec-trodes (i.e core without an active layer) with smooth

sur-faces.Fig 4shows SEM images of Pb layers plated during

charging of the cell at 20 mA cm−2for 1 h with an electrolyte

containing 1.5 M Pb(CH3SO3)2+ 0.9 M CH3SO3H and

var-ious concentrations of sodium ligninsulfonate Without

ad-ditive, the deposit can be seen to be made up of large

indi-vidual grains that are not totally merged and there are also

some deposit features growing out from the layer With

in-creasing additive concentration, the deposit becomes more

continuous and compact and the grains less angular Also,

even with the lowest ligninsulfonate concentration, there

is no sign of lead growing outside this layer towards the

counter electrode The positive influence of sodium lignin

sulfonate is even clearer when the edges of these deposits

are examined by obtaining images with the sample tilted

at 70◦, see Fig 5 In the absence of additive, the

individ-ual grains are clearly visible and some dendritic growth

is seen but with 1 g dm−3 lignin sulfonate, the deposit is

smoother and more compact.Fig 6illustrates the PbO2

de-posits formed in the same conditions and with low

concen-trations of the additive; both surfaces are almost featureless

on the 10␮m scale and the additive can be seen to have

rel-atively little effect With 5 g dm−3 ligninsulfonate, the

de-posit showed a number of hemispherical craters that were likely to result from oxygen bubbles damaging the deposit; this observation supports the postulate above that O2 evo-lution consumes a larger fraction of the charge when depo-sition occurs from an electrolyte with high ligninsulfonate concentrations

Further experiments examined the deposits on foam elec-trodes, nickel negative electrodes and reticulated vitreous carbon positive electrodes The battery was charged at

20 mA cm−2 for 1 h with an electrolyte containing 1.5 M

Pb(CH3SO3)2+ 0.9 M CH3SO3H, without and with 1 g dm−3

sodium ligninsulfonate.Fig 7shows SEM images of the Ni foam substrate and of the lead deposited onto the foam from solutions with and without 1 g dm−3ligninsulfonate Without

additive, some nodular growth is apparent particularly on the surface facing the positive electrode With the additive, the deposition is uniform throughout the foam structure.Fig 8

reports the SEM images of the lead dioxide on the positive electrode after a 6 h charge with the electrolyte containing sodium ligninsulfonate It can be seen that the deposition is uniform throughout the structure, forming over all exposed carbon surface and there is no sign of dendritic growth In fact, the deposition is largely uniform in the absence of the additive but the uniformity is further improved by the addition

of 1 g dm−3to the electrolyte.

Fig 4 SEM images of Pb layers plated onto a carbon powder/high density polyethylene composite plate (core without an active layer) from an electrolyte containing 1.5 M Pb(CH 3 SO 3 ) 2 + 0.9 M CH 3 SO 3 H and sodium ligninsulfonate: (a) 0 g dm −3, (b) 0.2 g dm−3, (c) 1 g dm−3and (d) 5 g dm−3 Flow cell[2],

mean linear flow rate: 2.5 cm s −1 One hour deposition at: 20 mA cm−2.

Fig 5 SEM images of the edges of the electrodes with sample tilted at 70 ◦: (a) 0 g dm−3and (b) 1 g dm−3sodium ligninsulfonate Other conditions as inFig 5.

Fig 6 SEM images of PbO deposits formed at the positive electrode during the experiments of Fig 5 : (a) 0 g dm −3and (b) 1 g dm−3sodium ligninsulfonate.

Fig 7 SEM of Pb on Ni foam Deposit formed by 1 h deposition at

20 mA cm −2 from 1.5 M Pb(CH

3 SO 3 ) 2 + 0.9 M CH 3 SO 3 H and sodium ligninsulfonate: (a) 0 g dm −3and (b) 1 g dm−3 Electrolyte mean linear flow

rate: 2.5 cm s −1.

The conclusion is clear—sodium ligninsulfonate

im-proves the quality and uniformity of the lead deposited on

the negative electrode during charging without detrimental

effects on the lead dioxide formed at the positive electrode

Several series of charge/discharge experiments were then

car-Fig 8 SEM image of PbO 2 formed on reticulated vitreous carbon positive electrode during a 6 h charge from 20 mA cm −2 from 1.5 M

Pb(CH 3 SO 3 ) 2 + 0.9 M CH 3 SO 3 H and 1 g dm −3sodium ligninsulfonate: (a)

surface facing negative electrode and (b) side view.

ried out without and with the additive using the electrolyte, 1.5 M Pb(CH3SO3)2+ 0.9 M CH3SO3H

Cells were constructed with two type 2 (reticulated vit-reous carbon) electrodes and an interelectrode gap of 4 mm The first was filled with electrolyte without sodium lignin-sulfonate and the intention was to charge at 20 mA cm−2

for 4 h but after about 3 h, shorting of the electrodes oc-curred; the negative electrode was examined by electron microscopy and as shown in Fig 1, dendrites were ob-served on the negative electrode Further experiments were then carried out with electrolyte containing 1 g dm−3sodium

ligninsulfonate using charge/discharge current densities of both 20 and 40 mA cm−2 No shorting occurred The

volt-age/time responses, seeFig 9, show an almost constant volt-age during charge (after an initial short period with a slightly lower value) and a voltage that decays only slowly during the discharge As expected, the overpotentials are slightly higher with the larger charge/discharge current densities The charge and energy efficiencies for these experiments are

re-Fig 9 Cell voltage vs time responses for cell with two type 2 (reticulated vitreous carbon electrodes) and 4 mm interelectrode gap Electrolyte: 1.5 M Pb(CH 3 SO 3 ) 2 + 0.9 M CH 3 SO 3 H + 1 g dm −3sodium ligninsulfonate + 1 g dm−3Ni(II) Mean linear flow rate: 10 cm s−1 Current densities: (a) 20 mA cm−2

and (b) 40 mA cm −2.

Table 1

The influence of ligninsulfonate on battery performance during 4 h charge/discharge experiments

Ligninsulfonate concentration (g dm −3) Current density (mA cm−2) Charge efficiency (%) Energy efficiency (%)

Cell with two type 2 (reticulated vitreous carbon electrodes) and 4 mm interelectrode gap Initial electrolyte: 1.5 M Pb(CH 3 SO 3 ) 2 + 0.9 M CH 3 SO 3 H Mean linear flow rate: 10 cm s −1.

ported inTable 1 It should be noted that at the higher

cur-rent density, the charge passed is 576 C cm−2

correspond-ing to a lead deposit equivalent to a uniform layer∼0.5 mm

thick

Another set of cells with two type 2 electrodes were

sub-jected to multiple, 1 h charge/discharge cycles at a current

density of 20 mA cm−2with electrolytes containing different

concentrations of ligninsulfonate Table 2 reports the

per-formance of the cells during the third cycle and it can be

seen that the additive has little effect at a 1 g dm−3level but

the higher concentration degraded performance significantly

due to both a decrease in charge efficiency and an increase in

overpotentials

Experiments involving 84, 15 min charge/discharge

cy-cles at 20 mA cm−2were carried out in a cell with a type

1 (nickel foam) negative electrode and a type 3 (scraped

reticulated vitreous carbon) positive electrode in electrolytes

Table 2

The influence of ligninsulfonate on battery performance during 1 h

charge/discharge experiments at 20 mA cm −2

Ligninsulfonate

concentration (g dm −3) Chargeefficiency (%) Energyefficiency (%)

Cell with two type 2 (reticulated vitreous carbon electrodes) and 16 mm

in-terelectrode gap Initial electrolyte: 1.5 M Pb(CH 3 SO 3 ) 2 + 0.9 M CH 3 SO 3 H.

Mean linear flow rate: 2.5 cm s −1.

with 0 and 1 g dm−3sodium lignin sulfonate.Fig 10reports

the voltage during the 1st–6th and 79th–84th cycles for the electrolyte containing additive and the early cycles may be compared with those reported previously for the electrolyte without additive (Fig 8 in[2]) The additive has little influ-ence on the overall form of the responses The presinflu-ence of

1 g dm−3ligninsulfonate does cause a small decrease in the

charge efficiency, seeTable 3, and the charge efficiency also declines very slightly with cycling The energy efficiency de-pends on the overpotentials as well as the charge efficiency and with these short cycles, the energy efficiency actually increases with cycling This results from the period of low overpotential at the beginning of the charge period that even-tually extends throughout the 15 min charge Unfortunately,

on longer charges, this overpotential period does not last; an

Table 3 The influence of ligninsulfonate and Ni(II) on battery performance during

900 s charge/discharge experiments at 20 mA cm −2

Ligninsulfonate concentration (g dm −3) Chargeefficiency (%) Energyefficiency (%)

The data are reported for 6th and 84th cycles Cell with two type 2 (reticulated vitreous carbon electrodes) and 16 mm interelectrode gap Initial electrolyte: 1.5 M Pb(CH 3 SO 3 ) 2 + 0.9 M CH 3 SO 3 H Mean linear flow rate: 2.5 cm s −1.

a Ni(II) added at a level of 1 g dm −3.

Fig 10 Voltage during the 1st–6th and 79th–84th charge/discharge cycles of a cell with a type 1 (nickel foam) negative electrode and a type 2 (reticulated vitreous carbon) positive electrode and interelectrode gap 4 mm Fifteen minutes charge/discharge cycles at 20 mA cm −2 Electrolyte: 1.5 M Pb(CH3SO3)2+ 0.9 M

CH 3 SO 3 H + 1 g dm −3sodium ligninsulfonate + 1 g dm−3Ni(II) Mean linear flow rate: 10 cm s−1.

increase in overpotential is clearly seen in the responses early

in the cycling This behaviour has been explained[1–3]by the

formation of some insoluble Pb(II) species remaining on the

positive electrode surface during the reduction of the PbO2

and this residue being easier to oxidize back to PbO2than

Pb2+in solution

3.1.2 Polyethylene glycol

Polyethylene glycol has also been recommended as an

ad-ditive for levelling lead deposits[7–9]and some preliminary

experiments were carried out with a low molecular weight

polyethylene glycol (2 0 0) It was found, however, that

addi-tions of up to 50 g dm−3polyethylene glycol had little

influ-ence on the cyclic voltammetry for the deposition of either

lead or lead dioxide (except for lower current densities

re-sulting from the higher viscosity of the medium) Moreover,

SEM images of the lead deposits showed little change with

even large additions of polyethylene glycol It was clear that

polyethylene glycol is not an effective levelling agent

dur-ing depositions from the methanesulfonic acid, battery

elec-trolyte

3.2 Additives for lead dioxide deposition/dissolution

On the basis of a literature that implies that the

chem-istry of PbO2 may be changed by the incorporation of

inorganic ions [10–14], Fe(III), Ni(II) and Bi(III) were

selected for study with the target of enhancing the

ki-netics of the PbO2/Pb(II) couple Cyclic voltammograms were recorded at a vitreous carbon disc electrode in 1.5 M Pb(CH3SO3)2+ 0.9 M CH3SO3H with additions of the metal ions in the range 0.1–10 g dm−3 These experiments showed

that the addition of Fe(III) and Bi(III) did not change the voltammetry significantly but the addition of Ni(II) led to

a decrease in nucleation overpotential for the deposition of PbO2 by up to 60 mV although the charge balance for the deposition/dissolution got worse Hence, it was considered worthwhile to carry out further experiments in the flow cell

in order to define the influence of Ni(II) on battery perfor-mance

In the first experiments, cells with type 1 (Ni foam) neg-ative electrode and type 3 (scraped reticulated vitreous car-bon) positive electrode and an electrolyte with various Ni(II) additions were subjected to 900 s charge/discharge cycles at

20 mA cm−2 Data taken from the 6th cycle is reported in

Table 4 It can be seen that even the lowest Ni(II) addition leads to a lower cell voltage during charge but little change

to the discharge voltage Unfortunately, a loss in charge ef-ficiency counter balances the lower charge voltage when the energy efficiency is calculated When the experiment with

1 g dm−3 Ni(II) was extended to 84 cycles the same

con-clusion results; the addition of the Ni(II) lowers the charge voltage but there is a decrease in the charge efficiency leading

to a very similar energy efficiency

Four hours charge/discharge experiments with an elec-trolyte containing both 1 g dm−3Ni(II) and 1 g dm−3sodium

Table 4

The influence of Ni(II) on battery performance during 900 s charge/discharge experiments at 20 mA cm −2

NiCO 3 (g dm −3) Average charge voltage (V) Average discharge voltage (V) Charge efficiency (%) Energy efficiency (%)

The data are reported for 6th cycles Cell with type 1 (Ni foam) negative electrode and type 3 (scraped reticulated vitreous carbon) positive electrodes and 4 mm interelectrode gap Initial electrolyte: 1.5 M Pb(CH SO ) + 0.9 M CH SO H Mean linear flow rate: 10 cm s −1.

ligninsulfonate were carried out at both 20 and 40 mA cm−2.

The cell voltage versus time responses shown inFig 9are

very similar to those in the absence of Ni(II) The same

so-lution was also used for an extended number of short cycles

The 1st–6th and 79th–84th cycles are shown inFig 10and

data from the experiments shown inTable 3 Again, in longer

timescale experiments, the advantage of adding Ni(II) is no

longer apparent

4 Conclusions

Sodium ligninsulfonate at a level of ≤1 g dm−3 is an

effective levelling agent for lead in methanesulfonic acid

media even when thick layers are deposited at relatively high

rates The need for levelling of the electroplated lead appears

to be greater with carbon as the substrate than when nickel is

used and, indeed, this is the reason that reticulated vitreous

carbon was used for many of the experiments in this paper

focusing on the quality of the deposit While leading to

more compact and dendrite free deposits, the ligninsulfonate

does adversely influence both the charge efficiency and the

overpotentials at both negative and positive electrodes to a

small extent, resulting in a slight decline in energy efficiency

during battery cycling In contrast, polyethylene glycol

was found to be ineffective as a levelling agent in battery

conditions

The attempts to use inorganic ions as catalysts for the

PbO2/Pb2+ couple were not fully successful While Ni(II)

does reduce the overpotential for charging the positive

elec-trode, at least on a short timescale, there is a parallel decrease

in charge efficiency that largely negates any improvement in

energy efficiency Catalysis of the PbO2/Pb2+couple remains

a target worthy of further attention

Acknowledgements

The authors would like to thank Regenesys Technolo-gies Ltd for financial support of the work and Dr Jon Cox

of Regenesys Technologies Ltd for the fabrication of the electrodes

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